A signal utilized in a transmitter, for example in a cellular telephone handset, is processed through various components that make up the transmitter before being transmitted. An average amplitude of the signal can vary widely, which in general affects the power and ultimately the quality of the signal that is sent over the transmitter.
In a transmitter of a handset utilizing, for example, wideband code division multiple access (WCDMA), the signal encompasses several physical channels at the physical layer of processing. The physical channels are combined in an orthogonal fashion and encoded to form a single complex data stream. The complex data stream is conventionally further processed by components in the transmitter, for example, pulse shaping and is further processed by a digital-to-analog converter (DAC) to form an analog signal that will transmitted over the transmitter.
The variation in the average amplitude of the signal can sometimes be quantified as an RMS (root mean square) level of the signal. To improve transmitter noise and carrier feed through performance, and hence the quality of the signal, the signal should be adjusted so that it covers the limits of the components in the transmitter, e.g., a digital-to-analog converter (DAC), without clipping of the signal.
For a typical channel configuration in accordance with various conventional standards, the variation in the signal due to channel configuration can be limited to 3 dB when measuring the RMS (root mean square) level. For example, under the standards established by the 3GPP (3rd Generation Partnership Project) Release 99/Release 4 in accordance with conventional practice, the variation in the RMS level of the signal is about 3 dB worst case. This is an amount that can be sufficiently absorbed in a line-up of components in a typical transmitter. Moreover, this amount does not tend to add an undue performance burden on other radio frequency integrated circuits, such as a modulator, voltage controlled amplifier or power amplifier, that are typically included in the line-up of components.
The addition, however, of the high speed dedicated physical control channel (HS-DPCCH), required for high speed downlink packet access (HSDPA) in accordance with the 3GPP (3rd General Partnership Project) Release 5, allows the variation in the RMS level of the signal to be about 8 to 9 dB worst case. This amount of variation is not efficiently absorbed in the line-up of components. Further releases may add additional channels, e.g., for HSUPA (high speed uplink packet access) which will likely result in further RMS level variation.
Conventional techniques provide for a fixed scaling of the signal, where the signal is simply scaled up; usually such scaling is limited so that the maximum amplitude signal that is expected is not clipped at the digital-to-analog converter (DAC) component. In conventional techniques, however, a signal having a minimum amplitude under normal operating conditions will have suboptimal signal levels that are well below the maximum that can be supported in the DAC components.
Unfortunately, with suboptimal signal levels driving the radio frequency modulator, the carrier feed through and radio frequency signal-to-noise ratio of the modulator tend to be degraded approximately dB for dB. The components included in the radio frequency circuitry of a conventional line-up of components can incur significant additional complexity to allow for carrier feed through mitigation and current drain for noise improvements, so that the overall transmitter system can comply with minimum standards.